This invention relates to a method of cleaning stator water cooling systems that can be used to cool stator bars contained in the generators of electrical power plants.
In electrical power plants, high density current is passed through generator stators, thus causing the stators to be heated. In order to control this heating, the stators are constructed of a network of stator strands in which hollow cooper conductors are placed, through which circulates cooling water. Cooling water may be provided by a stator water cooling system (SWCS) connected to the generator. A typical SWCS connected to a generator is shown in FIG. 1. As shown in FIG. 1, a typical SWCS 2 is composed of many parts. First there is a high-temperature area of the SWCS 8 containing in part stator bars, rectifiers and reservoir tanks. The high-temperature area of a typical SWCS is the region of the SWCS into which the water flows from the generator prior to contacting the heat exchanger 4. After the water leaves the high-temperature area, it passes through heat exchangers and into the low-temperature area 5 of the SWCS. The low-temperature area of a typical SWCS contains in part, make-up water systems 6 and 7. A typical SWCS is connected to a generator 3 by connectors 1.
The formation of deposits on the internal walls of the hollow copper conductors is a frequently encountered problem. This formation of copper oxide (CuO) deposits causes numerous problems in electrical power plants. Not only can it lead to obstruction of the hollow copper conductors, but the heated water in the copper conductors can partially dissolve the CuO deposits until saturation is achieved, thereby providing a saturated aqueous solution of copper oxide to the SWCS. Additionally, fine particulates of undissolved copper oxide, deposited by the cooled water in the low-temperature area of the SWCS can be removed from the low-temperature area interior piping material surfaces by the subsequent flow of water and transported into the strainers of the SWCS. The SWCS pumps this saturated, aqueous solution of copper oxide through the low-temperature section of the SWCS (the section located after the heat exchangers and before the connecting means to the generator) and into the strainers.
This saturated aqueous solution of copper oxide passes through strainers in the low-temperature section of the SWCS. CuO particles are deposited on these strainers by the aqueous solution thus facilitating the growth of copper oxide crystals on the strainer, thereby leading to the strainer""s plugging and substantially impeding the flow of water to the generator. Growth of CuO crystals on the strainers leads to decreased flow of water through the generator, thereby causing a heating of the generator""s stators and in the long run, to numerous extended generator down-times or to decreases in charging.
One attempted solution to this problem has used on-line demineralization circuits. This technique, however, uses additional expensive ion-exchange resins to remove the copper cations from the water. Other attempted solutions require the complete system (the SWCS and the generator) to be periodically stopped and the copper conductors cleaned using acid solutions and/or complexing solutions, thereby leading to increased corrosion rates in the hollow copper conductor tubes. All of the current methods of removing CuO deposits from SWCS contain the additional undesired feature of removing the protective CuO layer from the stator strands, thereby risking the development of unstable CuO surfaces. Unstable CuO surfaces on the stator strands promote the development of the CuO deposits in the low temperature area of the SWCS thereby leading to an increase in the plugging rate for SWCS strainers.
Thus, there exists a need for a method that makes it possible to control the formation and/or allows the elimination of deposits responsible for the obstruction of the strainers in stator water cooling circuits operated in aerated mode while maintaining the protective CuO oxide layer on the stator water cooling strands. Such a method must not result in an extended down-time of the generator, must not require expensive demineralization circuits, must not generate high conductivities at the cooling water level and must not cause a high degree of corrosion of the hollow copper pipes.
This invention is directed to a method of removing deposits from stator water cooling system piping thereby preventing strainer plugging. In particular, this method removes the copper oxide deposits from stator water cooling system piping surfaces, thereby eliminating the source of particulates that enable nucleation of crystal growth on strainer mesh.
The method of the invention removes copper oxide deposits from SWCS piping material surfaces by disconnecting the generator from the SWCS and connecting bridging piping to the SWCS at the points from which the generator was disconnected, allowing the SWCS to operate without the generator. Then, a sufficient amount of an agent that dissolves or removes copper oxide deposits from said piping material surfaces is flowed through the SWCS for a time sufficient to dissolve or remove substantially all copper oxide deposits without removing the protective oxide coating from the water-cooled stator strands. Finally, the piping material surfaces are rinsed with sufficient water until the output water from the rinsing of the piping material surfaces is essentially neutral. Preferably, the agent is an acidic aqueous solution.